Phomopsis subordinaria field survey
We found that P. subordinaria was widely spread in P.
lanceolata populations in the Åland Islands as in 47.7% (124) of the
sampled 260 populations; one or more infected plants were found (Figure
1A). Large populations were more commonly infected than small, and host
population connectivity was positively associated with P.
subordinaria infection (Table 1, Figs. 1B-C). We also found regional
spatial variation in infection prevalence that was the highest in
Hammarland, where 67.5% of the 37 surveyed host populations were
infected, and lowest in Saltvik where 31.6% of the 19 surveyed host
populations were infected (Table 1, Figs. 1A).
Within infected host populations, P. subordinaria population size
and infection prevalence were typically low. In nearly half of the
(48.4%) populations, we found 1-10 infected plants (Figure 1E). In
36.3% of the infected populations 11-100 infected plants were found,
and in only 1.6% populations we found more than 1000 infected
individuals (Figure 1E). Within
populations less than 10% of the plants were infected in the majority
of infected populations (87.9%), and in only 2.4% of the populations
we observed more than 25% infected plants (Figure 1F). At the plant
level, we found that P. subordinaria infections were highly
virulent. On average 84.9% of stems were infected within infected
plants (Figure 1D).
When we tested factors affecting pathogen within-population prevalence,
regional districts differed in the proportion of plants infected and
within-host infection load measured as proportion of infected stalks had
small but significant positive effect on within-population prevalence
(Table 1; Fig 1G). Neither host population connectivity nor size
influenced P. subordinaria within-population prevalence (Table
1). None of the tested variables - regional district, host population
connectivity and size, P. subordinaria epidemic size, or
within-population prevalence – had a significant effect on within-host
infection load (Table 1).